In a mobile communication system, a multicarrier technology has fully grown and developed gradually in recent years. Since a multicarrier clipping technology may reduce a peak to average rate of a multicarrier signal so as to raise efficiency of a high power amplifier and cut the cost of power amplification equipment. For this reason, the multicarrier clipping technology has seen used more and more.
A multicarrier clipping technique based on a multi-band bandpass filter and a filter coefficient selection has been proposed in the prior art with two embodiments adopting a real coefficient filtering and a complex coefficient filtering respectively. As shown in FIG. 1a, a multicarrier signal clipping device 60 adopting a real coefficient filtering in the above-mentioned application document includes a noise generation unit 600, a positive frequency shift unit 601, an amplitude prediction unit 602, a noise amplitude adjustment unit 603, a wideband noise frequency shift processing unit 604, a reverse frequency shift unit 605, a lowpass filtering unit 606, a peak value counteracting unit 607, a clipping filter selection unit 608, and a subcarrier power detection unit 609, and the noise generation unit 600 includes a squarer 6001, a squarer 6002 and a noise generator 6003.
In the multicarrier clipping structure 60, the noise generation unit 600 first receives an intermediate frequency multicarrier joint signal to generate a clipped noise, then sends the clipped noise to the positive frequency shift unit 601, and the positive frequency shift unit 601 moves the clipped noise received to a positive frequency section to get a positive clipped noise. Meanwhile, the subcarrier power detection unit 609 detects the magnitude of each subcarrier power, calculates the power to see if it falls, and further calculates magnitude of the falling if the power falls; the clipping filter selection unit 608 selects an appropriate clipping filer coefficient according to the power information from the subcarrier power detection unit and sends the selected coefficient to the wideband noise frequency shift processing unit 604 and the amplitude prediction unit 602; the wideband noise frequency shift processing unit 604 updates the filter coefficient according to these coefficients. Then, the positive frequency clipped noise is simultaneously sent to the amplitude prediction unit 602 and the noise amplitude adjustment unit 603. The amplitude prediction unit 602 makes a prediction for the amplitude of the positive frequency clipped noise which passes an actual clipping filter and then sends the predicted value to the noise amplitude adjustment unit 603. The noise amplitude adjustment unit 603 performs an amplitude adjustment for the clipped noise according to the predicted value to enable the peak value of the adjusted signal after being filtered to be closer to the positive frequency clipped noise before the adjustment. The wideband noise frequency shift processing unit 604 receives the noise signal after the amplitude adjustment and shapes the spectrum thereof to bring enough suppression to a transition band and a block band of the spectrum thereof, and then the reverse frequency shift unit 605 moves the noise signal from the wideband noise frequency shift processing unit 604 back to the original frequency section. The filtering noise after being reversely frequency shifted then re-passes the lowpass filtering unit 606 to filter the unnecessary negative frequency noise component; eventually a peak value counteracting is performed for the original multicarrier joint signal in the peak value counteracting unit 607, in other words, the noise signal after being processed with the amplitude adjustment and the spectrum shaping is imposed on the original and postponed multicarrier joint signal so as to get the peak value of the original multicarrier joint signal well suppressed. This technique makes a selection of the clipping filter coefficient according to the subcarrier power so as to prevent such indexes as a small power carrier Peak Code Domain Error (PCDE) and an Error Vector Magnitude (EVM) from worsening.
As shown in FIG. 1b, a multicarrier signal clipping device 70 adopting a complex coefficient filting includes a noise generation unit 700, an amplitude prediction unit 701, a noise amplitude adjustment unit 702, a complex filtering wideband noise processing unit 703, a peak value counteracting unit 704, a clipping filter selection unit 705, and a subcarrier power detection unit 706. The noise generation unit 700 includes a squarer 7001, a squarer 7002 and a noise generator 7003. The structures and functions of the noise generation unit 700, the amplitude prediction unit 701, the noise amplitude adjustment unit 702, the peak value counteracting unit 704, the clipping filter selection unit 705 and the subcarrier power detection unit 706 are the same as those of corresponding units in the multicarrier signal clipping device 60 adopting the real coefficient filtering.
In accordance with the method corresponding to the device 70, the noise generation unit 700 first receives a multicarrier joint signal and generates a clipped complex noise, then sends the clipped complex noise simultaneously to the amplitude prediction unit 701 and the noise amplitude adjustment unit 702. Then, the subcarrier power detection unit 706 detects the magnitude of each subcarrier power, detects whether the power falls, and further calculates the amplitude of the falling if the power falls. The clipping filter selection unit 705 selects an appropriate clipping filter coefficient according to the power information from the subcarrier power detection unit 706 and sends these selected coefficients to the amplitude prediction unit 701 and the complex wideband noise processing unit 703. The amplitude prediction unit 701 performs a prediction for the amplitude of the clipped complex noise which passes the actual clipping complex filter and then sends the predicted value to the noise amplitude adjustment unit 702. The noise amplitude adjustment unit 702 performs an amplitude adjustment for the clipped noise according to the predicted value to make the peak value of the adjusted signal after being filtered more close to the original clipped complex noise. The complex filtering wideband noise processing unit 703 receives the complex noise signal after being processed with the amplitude adjustment, shapes the spectrum thereof to make the spectrum thereof meet a certain requirement, and then the peak value counteracting unit 704 performs the peak value counteracting of the original multicarrier joint signal.
In order to use a locating method of an Observed Time Difference Of Arrival (OTDOA) in a Wideband Code Division Multiple Access (WCDMA) system, a Third Generation Packet Protocol (3GPP) requires that base stations support an Idle Period in DownLink (IPDL) mechanism. In the IPDL mechanism, each base station will interrupt all the downlink transmitting signals in the base station, including those for a common channel and a dedicated channel, for a rather short duration such as half a slot or one slot, and the interruption is called an IPDL. During the IPDL, a User Equipment (UE) needing to be located in the base station cell measures signals from other base stations. Corresponding network measuring unit such as a Locating/Measuring Unit (LMU) fulfills the measurement of a Reaching Time Difference (RTD), and obtains the differences among the times at which different base station signals reach the UE. Then the UE location may be calculated according to a plurality of the time differences. The 3GPP specifies the base station output power limit during these idle periods in an IPDL time template mode. As shown in FIG. 2, the lowest demands of the IPDL time template are that the measured value of the signal's average power should be no larger than the base station's maximum output power −35 dB during the time period between 27 chips after the IPDL period starts and 27 chips before the IPDL period ends.
However, in the existing multicarrier clipping technology, as object of the bandpass filtering is a wideband signal, the IPDL period is rather short in general, and a starting moment for the IPDL is not fixed. When the IPDL starts in a subcarrier, the clipped signals of other subcarriers will be leaked into the frequency band where the subcarrier starting the IPDL is located so that an extra-large power of the subcarrier signal is generated after being clipped, which is unable to meet demands of the IPDL time template and further affects the locating function of the base stations using the IPDL mechanism.